The present invention relates generally to the field of laser diodes, and more particularly to architecture for short-wavelength nitride based laser diode arrays.
Short-wavelength nitride based laser diodes provide smaller spot size and a better depth of focus than red and infrared (IR) laser diodes for laser printing operations and other applications. Single-spot nitride laser diodes have applications in areas such as optical storage.
Laser diode arrays are desirable for application to high-speed laser printing. Printing at high speeds and at high resolution requires laser arrays due to the fundamental limits of polygon rotation speed, laser turn-on times and laser power. Laser diode arrays have previously been employed using red and infrared laser diode structures. Dual-spot red lasers and quad-spot infrared lasers have been used for laser printers.
Laser diodes based on higher bandgap semiconductor alloys such as AlGalnN have been developed. Excellent semiconductor laser characteristics have been established in the near-UV to violet spectrum, principally by Nichia Chemical Company of Japan. See for example, A. Kuramata et al., xe2x80x9cRoom-temperature CW operation of InGaN Laser Diodes with a Vertical Conducting Structure on SiC Substratexe2x80x9d, Japanese Journal of Applied Physics, Vol. 37, L1373 (1998), S. Nakamura et al., xe2x80x9cCW Operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substratesxe2x80x9d, Applied Physics Letters, Vol. 72(6), 2014 (1998) and S. Nakamura and G. Fasol, xe2x80x9cThe Blue Laser Diode-GaN based Light Emitters and Lasersxe2x80x9d, (Springer-Verlag, 1997) all of which are incorporated by reference in their entirety.
Extension of dual-spot lasers to shorter wavelengths enables printing at higher resolution. The architecture for short- wavelength laser diode arrays has needed to be different when nitride based laser diodes are used in arrays because mirrors need to be formed by dry etching instead of cleaving and nitride based devices are mostly grown on insulating substrates such as sapphire.
A group from the University of California has developed a technique for separation of GaN films from sapphire substrates using a UV-excimer laser. The University of California technique uses an ultraviolet excimer laser to decompose a thin portion of the GaN layer at the interface with the sapphire substrate. By proper adjustment of the excimer laser flux, the interfacial GaN is decomposed into Ga and N with minimal damage. Subsequently, the GaN film is removed by gentle heating of the remaining Ga metal which has a melting point of 30xc2x0 C. at the film-substrate interface. See W. S. Wong et al., xe2x80x9cDamage-free separation of GaN thin films from sapphire substratesxe2x80x9d, Applied Physics Letters, Vol. 72, 599 (1998) which is incorporated by reference in its entirety.
Architectures using insulating substrates allow the economical construction of nitride based quad-spot diode laser and surface-emitting dual-quad-spot laser diode arrays. Currently, most advanced nitride based single laser structures are grown on insulating sapphire (Al2O3) substrates. The use of insulating substrates for laser diode arrays presents a special problem in providing electrical contacts for the laser diodes. In contrast to the situation where conducting substrates are used, insulating substrates cannot provide a common backside contact for all laser diodes in an array. Hence, providing electrical contacts to laser diode arrays on insulating substrates has required the use of special architectures.
Removal of the insulating substrate after growth of the laser diode array structures simplifies providing electrical contacts to the laser diode arrays and avoids special architectures while allowing a superior heat sink to be attached to the laser diode arrays. The laser diode array may be attached to a thermally conductive wafer before or after substrate removal by soldering, thermo-compression bonding or other means. Attachment of the thermally conductive substrate after removal of the insulating substrate requires the attachment of a support substrate as an intermediate step. Attachment of the thermally conductive wafer to the laser diode array before removal of the insulating substrate allows positioning of the thermally conductive substrate on the side of the laser diode array closer to the laser active region for more effective heat sinking than if the laser diode array is attached to the thermally conductive substrate after removal of the insulating substrate. If the nitride laser membrane is properly aligned during the attachment process with the thermally conductive substrate, cleaved mirror facets may be formed. Cleaved rather than etched mirror facets result in perfectly parallel, vertical, and smooth mirrors.